Transistors can be divided into two main types: bipolar junction transistors and field-effect transistors. Both types share a common structure comprising three electrodes with a semiconductive material disposed therebetween in a channel region. The three electrodes of a bipolar junction transistor are known as the emitter, collector and base, whereas in a field-effect transistor the three electrodes are known as the source, drain and gate. Bipolar junction transistors may be described as current-operated devices as the current between the emitter and collector is controlled by the current flowing between the base and emitter. In contrast, field-effect transistors may be described as voltage-operated devices as the current flowing between source and drain is controlled by the voltage between the gate and the source.
Transistors can also be classified as p-type and n-type according to whether they comprise semiconductive material which conducts positive charge carriers (holes) or negative charge carriers (electrons) respectively. The semiconductive material may be selected according to its ability to accept, conduct, and donate charge. The ability of the semiconductive material to accept, conduct, and donate holes or electrons can be enhanced by doping the material. The material used for the source and drain electrodes can also be selected according to its ability to accept and inject holes or electrons. For example, a p-type transistor device can be formed by selecting a semiconductive material which is efficient at accepting, conducting, and donating holes, and selecting a material for the source and drain electrodes which is efficient at injecting and accepting holes from the semiconductive material. Good energy-level matching of the Fermi-level in the electrodes with the HOMO (Highest Occupied Molecular Orbital) level of the semiconductive material can enhance hole injection and acceptance. In contrast, an n-type transistor device can be formed by selecting a semiconductive material which is efficient at accepting, conducting, and donating electrons, and selecting a material for the source and drain electrodes which is efficient at injecting electrons into, and accepting electrons from, the semiconductive material. Good energy-level matching of the Fermi-level in the electrodes with the LUMO (Lowest Unoccupied Molecular Orbital) level of the semiconductive material can enhance electron injection and acceptance.
Transistors can be formed by depositing the components in thin films to form thin-film transistors. When an organic material is used as the semiconductive material in such a device, it is known as an organic thin-film transistor (OTFT).
Various arrangements for OTFTs are known. One such device is an insulated gate field-effect transistor which comprises source and drain electrodes with a semiconductive material disposed therebetween in a channel region, a gate electrode disposed over the semiconductive material and a layer of insulting material disposed between the gate electrode and the semiconductive material in the channel region.
The conductivity of the channel can be altered by the application of a voltage at the gate. In this way the transistor can be switched on and off using an applied gate voltage. The drain current that is achievable for a given voltage is dependent on the mobility of the charge carriers in the organic semiconductor in the active region of the device (the channel region between the source and drain electrodes). Thus, in order to achieve high drain currents with low operational voltages, organic thin-film transistors must have an organic semiconductor which has highly mobile charge carriers in the channel region.
High mobility OTFTs containing “small molecule” organic semiconductor materials have been reported, and the high mobility has been attributed, at least in part, to the highly crystalline nature of the small molecule organic semiconductors in the OTFT. Particularly high mobilities have been reported in single crystal OTFTs wherein the organic semiconductor is deposited by thermal evaporation. A single crystal OTFT is disclosed in Podzorov et al, Appl. Phys. Lett. 2003, 83(17), 3504-3506.
Formation of the semiconductor region by solution deposition of a blend of a small molecule organic semiconductor and a polymer is disclosed in Smith et. al., Applied Physics Letters, Vol 93, 253301 (2008); Russell et. al., Applied Physics Letters, Vol 87, 222109 (2005); Ohe et. al., Applied Physics Letters, Vol 93, 053303 (2008); Madec et. al., Journal of Surface Science & Nanotechnology, Vol 7, 455-458 (2009); Kang et. al., J. Am. Chem. Soc., Vol 130, 12273-75 (2008); Chung et al, J. Am. Chem. Soc. (2011), 133(3), 412-415; Lada et al, J. Mater. Chem. (2011), 21(30), 11232-11238; Hamilton et al, Adv. Mater. (2009), 21(10-11), 1166-1171; and WO 2005/055248.
Reducing contact resistance at the source and drain electrodes is disclosed in WO 2009/000683 by selectively forming a self-assembled layer of a dopant for the organic semiconductor on the surface of the source and drain electrodes.